The solubility of fish-produced high magnesium calcite in seawater

Fish have been shown to produce high (10 to 48 mol %) magnesium calcite as part of the physiological mechanisms responsible for maintaining salt and water balance. The importance of this source to the marine carbon cycle is only now being considered. In this paper, we report the first measurements of the solubility of this CaCO3 in seawater. The resulting solubility (pK(sp)* = 5.89 +/- 0.09) is approximately two times higher than aragonite and similar to the high magnesium calcite generated on the Bahamas Banks (pK(sp)* = 5.90). This high solubility of fish-produced CaCO3 is a result of the high magnesium content and not a product of micro-environments created by microbial activity. This material is soluble in near surface waters, contributing to the input of carbonate to surface ocean waters, and may at least partially explain the observed increase in total alkalinity above the aragonite saturation horizon.